The invention relates to a process for the removal of m-chlorotoluene from chlorotoluene mixtures with a content of up to 10% by weight of m-chlorotoluene, with reference to the total amount of chlorotoluene in the mixture.
Chlorotoluenes in isomerically pure form are valuable intermediates for pharmaceuticals and agricultural chemicals (Ullmans Enzyklopadie der technischen Chemie, [Ullmann's Encylopaedia of industrial chemistry], 4th edition, volume 9, page 514). The requirement on the purity of the chlorotoluenes is high, particularly in the area of active substances.
A process for the extraction of m-chlorotoluene has been disclosed in U.S. Pat. No. 3,655,783, in which m-chlorotoluene is extracted from a mixture of dichlorotoluenes by addition of HF/BF.sub.3 and an organic solvent. The disadvantages connected with the addition of the fluorine compounds, such as the employment of expensive materials for the prevention of damage by corrosion and high expenditure on apparatus for safe handling of the HF/BF.sub.3 mixture, are obvious. The addition of a solvent, which considerably complicates later working up by distillation, is also disadvantageous.
A process for the separation of mixed monochlorotoluenes by adsorption on a column filled with zeolite and subsequent desorption with readsorbants has been disclosed in EP-PS (European Patent Specification) No. 0,099,161. However, in this process the very high technical complexity with which the separation is realized is disadvantageous. Additional solvents, such as toluene or chlorobenzene, are required, so that additional distillations are necessary for working up the mixture.
Furthermore, separation of a mixture of the three chlorotoluene isomers by initial fractionation and removal of the m-chlorotoluene remaining in the p-chloro-toluene by multistage crystallization of the melt is known (Ullmans Enzyklopadie technischen Chemie [Ullman's Encyclopaedie of industrial chemistry], 4th edition, volume 9, page 513.) The high technical complexity and the considerable losses of p-chlorotoluene, which in contrast to o-chlorotoluene is a substantially more valuable chlorotoluene isomer, are disadvantageous.